Yup - doesn't work. I tried all the same things as OP, and also checked the ini files to make sure it was loading full screen. I've just verified the cache and 1 item didn't verify, which is redownloading (3.8gb). Hopefully that'll fix it.

In the skyrim launcher, go to data files and find the high resolution pack, and uncheck it.

I've tried unticking absolutely everything in there, but it doesn't make a difference. I've also tried dropping all settings to the lowest, deleting and redownloading the entire game again and updating graphics drivers to the latest beta ones. All to no avail.

I'm wondering if the mod to have borderless windows will act as a workaround.

Should also add, that when I first launch the game it is full screen for about half a second during the logo, then it flashes to the sort of windowed mode with the borders.